This invention generally relates to reconfigurable arrays of sensors (e.g., optical, thermal, pressure, ultrasonic). In particular, the invention relates to reconfigurable micromachined ultrasonic transducer (MUT) arrays. One specific application for MUTs is in medical diagnostic ultrasound imaging systems. Another specific example is for non-destructive evaluation (NDE) of materials, such as castings, forgings, or pipelines.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducers that are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is continuously refocused along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducers arranged in one or more rows and driven with separate voltages in transmit. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducers in a given row can be controlled to produce ultrasonic waves that combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused in a selected zone along the beam.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducers are summed so that the net signal is indicative of the ultrasound reflected from a single focal zone in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer. The time delays are adjusted with increasing depth of the returned signal to provide dynamic focusing on receive.
The quality or resolution of the image formed is partly a function of the number of transducers that respectively constitute the transmit and receive apertures of the transducer array. Accordingly, to achieve high image quality, a large number of transducers is desirable for both two- and three-dimensional imaging applications. The ultrasound transducers are typically located in a hand-held transducer probe that is connected by a flexible cable to an electronics unit that processes the transducer signals and generates ultrasound images. The transducer probe may carry both ultrasound transmit circuitry and ultrasound receive circuitry.
A reconfigurable ultrasound array is one that allows groups of subelements to be connected together dynamically so that the shape of the resulting element can be made to match the shape of the wave front. This can lead to improved performance and/or reduced channel count. Reconfigurability can be achieved using a switching network.
Recently semiconductor processes have been used to manufacture ultrasonic transducers of a type known as micromachined ultrasonic transducers (MUTs), which may be of the capacitive (MUT) or piezoelectric (pMUT) variety. MUTs are tiny diaphragm-like devices with electrodes that convert the sound vibration of a received ultrasound signal into a modulated capacitance. For transmission the capacitive charge is modulated to vibrate the diaphragm of the device and thereby transmit a sound wave. One advantage of MUTs is that they can be made using semiconductor fabrication processes, such as microfabrication processes grouped under the heading “micromachining”. The systems resulting from such micromachining processes are typically referred to as “micromachined electromechanical systems (MEMS). As explained in U.S. Pat. No. 6,359,367:                Micromachining is the formation of microscopic structures using a combination or subset of (A) Patterning tools (generally lithography such as projection-aligners or wafer-steppers), and (B) Deposition tools such as PVD (physical vapor deposition), CVD (chemical vapor deposition), LPCVD (low-pressure chemical vapor deposition), PECVD (plasma chemical vapor deposition), and (C) Etching tools such as wet-chemical etching, plasma-etching, ion-milling, sputter-etching or laser-etching. Micromachining is typically performed on substrates or wafers made of silicon, glass, sapphire or ceramic. Such substrates or wafers are generally very flat and smooth and have lateral dimensions in inches. They are usually processed as groups in cassettes as they travel from process tool to process tool. Each substrate can advantageously (but not necessarily) incorporate numerous copies of the product. There are two generic types of micromachining . . . 1) Bulk micromachining wherein the wafer or substrate has large portions of its thickness sculptured, and 2) Surface micromachining wherein the sculpturing is generally limited to the surface, and particularly to thin deposited films on the surface. The micromachining definition used herein includes the use of conventional or known micromachinable materials including silicon, sapphire, glass materials of all types, polymers (such as polyimide), polysilicon, silicon nitride, silicon oxynitride, thin film metals such as aluminum alloys, copper alloys and tungsten, spin-on-glasses (SOGs), implantable or diffused dopants and grown films such as silicon oxides and nitrides.The same definition of micromachining is adopted herein.        
The cMUTs are usually hexagonal-shaped structures that have a membrane stretched across them. This membrane is held close to the substrate surface by an applied bias voltage. By applying an oscillatory signal to the already biased cMUT, the membrane can be made to vibrate, thus allowing it to radiate acoustical energy. Likewise, when acoustic waves are incident on the membrane the resulting vibrations can be detected as voltage changes on the cMUT. A cMUT cell is the term used to describe a single one of these hexagonal “drum” structures. The cMUT cells can be very small structures. Typical cell dimensions are 25–50 microns from flat edge to flat edge on the hexagon. The dimensions of the cells are in many ways dictated by the designed acoustical response. It may not be possible to create larger cells that still perform well in terms of frequency response and sensitivity desired.
Unfortunately, it is difficult to produce electronics that would allow individual control over such small cells. While in terms of the acoustical performance of the array as whole, the small cell size is excellent and leads to great flexibility, control is limited to larger structures. Grouping together multiple cells and connecting them electrically allows one to create a larger subelement, which can have the individual control while maintaining the desired acoustical response. So a subelement is a group of electrically connected cells that cannot be reconfigured. For the purpose of this disclosure, the subelement is the smallest independently controlled acoustical unit. One can form rings or elements by connecting subelements together using a switching network. The elements can be reconfigured by changing the state of the switching network. However, subelements comprise connected cells that are not switchably disconnectable and thus cannot be reconfigured. All of the following analysis is also valid if the array is made of PZT or some other more common or future transducer technology.
Reconfigurability using silicon-based ultrasound transducer subelements was described in U.S. patent application Ser. No. 10/383,990. One form of reconfigurability is the mosaic annular array, also described in that patent application. The mosaic annular array concept involves building annular elements by grouping subelements together using a reconfigurable electronic switching network. The goal is to reduce the number of beamforming channels, while maintaining image quality and improving slice thickness. To reduce system channels, the mosaic annular array makes use of the fact that for an unsteered beam, the delay contours on the surface of the underlying two-dimensional transducer array are circular. In other words, the iso-delay curves are annuli about the center of the beam. The circular symmetry of the delays leads to the obvious grouping of those subelements with common delays and leads to the annular array concept. The reconfigurability can be used to step the beam along the larger underlying two-dimensional transducer array in order to form a scan or image. The reconfigurability might also be used to improve performance for multiple transmit applications by assigning more channels to the smaller active aperture in the near field. There are many other applications where reconfigurability might prove useful.
In a mosaic annular transducer array and other mosaic transducer arrays, a large number of ultrasound transducer subelements must be connected together using a distributed switch matrix. The subelements build up larger elements that are used for transmission and reception of ultrasound signals. The configuration of the elements and therefore the subelements changes each time that a new line of data or “view” is acquired. Each time that the configuration changes, the state (on or off) of all of the switches in the switching matrix must be updated to create the required interconnections that build up the new state of the elements and subelements.
In a reconfigurable sensor array, a large number of sensor subelements must be accessed by system electronics. This presents a significant bottleneck in terms of routing of signal and control lines to associated system processing electronics.
In current high-channel-count systems, connections to individual sensor elements are brought out using individual flexible wires and routed to external printed circuit boards housing the necessary scanning electronics. The wiring and printed circuit boards are bulky and not currently applicable to a very large number of transducer subelements as is the case in a mosaic transducer array.
Reconfigurable ultrasound arrays require a complex switching network that may be difficult or impossible to implement with currently available electronics. There is a need for a simplified switching network that has application in arrays of ultrasonic transducer subelements as well as in arrays of other types of sensors (e.g., optical, thermal, pressure). There is also a need for a construction comprising integrated switching electronics disposed beneath the sensor array for rapidly reconfiguring the sensor array.